1. Field of the Invention
The present invention relates to a water-proof sound-transmitting membrane to be used for electrical appliances with a sound function, and a method for producing the water-proof sound-transmitting membrane. The present invention also relates to an electrical appliance including the water-proof sound-transmitting membrane.
2. Description of Related Art
Electrical appliances, such as cellular phones, notebook computers, electronic notebooks, digital cameras, and game machines, are desired to have a water-proof structure because they often are used outdoors. Parts that are most difficult to have the water-proof structure in an electrical appliance are a sound emitting part and a sound receiving part, such as a speaker, a microphone, and a buzzer. Generally, a housing of the electrical appliance with the sound function has openings at positions corresponding to the sound emitting part and the sound receiving part. Sound is transmitted between the sound emitting part and outside and between the sound receiving part and the outside through these openings.
A water-proof sound-transmitting membrane is known as a member for preventing water from entering into the housing through the openings provided for the sound emitting part and the sound receiving part while ensuring a satisfactory sound transmittance. The water-proof sound-transmitting membrane is a thin film made of a material that hardly hinders the transmittance of sound. By closing the openings of the housing with the water-proof sound-transmitting membrane, it is possible to achieve both the sound transmittance and the waterproofness at these openings. As the water-proof sound-transmitting membrane, a polytetrafluoroethylene (PTFE) porous membrane preferably is used (See JP 2004-83811 A).
When the average pore diameter of the PTFE porous membrane is reduced, the waterproofness of the membrane is enhanced, but on the other hand, the surface density of the membrane is increased and the sound transmittance is lowered. That is, the waterproofness and the sound transmittance of the water-proof sound-transmitting membrane are in a trade-off relationship. Therefore, it is not easy to enhance the waterproofness without lowering the sound transmittance. In JP 2004-83811 A, both the waterproofness and the sound transmittance are achieved by specifying the average pore diameter and the surface density of the PTFE porous membrane.
In recent years, the level of the waterproofness required for electrical appliances has become higher every year. Specifically, electrical appliances are required to have a waterproofness of not only a level that allows them to be used in daily lives but also a level that allows them to be immersed in water, and furthermore, a level that allows them to be used at a predetermined depth in water for a certain period of time. However, it is not assumed that the water-proof sound-transmitting membrane disclosed in JP 2004-83811 A is used in the situation in which an electrical appliance is immersed in water.
JP 7(1995)-292144 A discloses a method for producing, although not a water-proof sound-transmitting membrane, a PTFE complex porous membrane for a high-efficiency air filter (a so-called ULPA filter or HEPA filter) used for removing fine particles in the air. In the production method (see CLAIMS) disclosed in JP 7(1995)-292144 A, a mixture containing a PTFE powder obtained by an emulsion polymerization method and a liquid lubrication agent is formed into a PTFE film by paste extrusion first. Next, the obtained film is stretched in the direction of the extrusion (a longitudinal direction, that is, an MD direction) at a temperature equal to or lower than a melting point of a sintered PTFE product, thereby forming a complex body composed of at least two stretched PTFE films stacked on each other. Subsequently, the obtained complex body is stretched in a width direction (a direction perpendicular to the longitudinal direction, that is, a TD direction), thereby forming a complex body that has been stretched in the longitudinal direction and the width direction so that an areal stretching ratio thereof is at least 50. Finally, this complex body is subject to heat setting, if needed, to obtain a high-efficiency air filter. JP 7(1995)-292144 A also describes that the above-mentioned production method makes it possible to obtain a PTFE porous membrane with a low pressure loss (with a high gas permeability), a small pore diameter, and extremely few defects such as pinholes. In addition, JP 7(1995)-292144 A describes that this porous membrane is suitable for a high-efficiency air filter (See [0014]).